GStreamer 1.0: fix some problems with QGstVideoRendererSink.

GStreamer 1.0: fix some problems with QGstVideoRendererSink.

- Correctly free resources on deletion.
- Correctly stop the sink. We were stopping only when null caps
  were passed to set_caps() but that doesn't seem to always happen.
  Implement GstBaseSink.stop() which is always and consistently
  called by GStreamer.
- Remove pre-roll support (as done previously for 0.10, see
  commit 3b20608f).

Change-Id: I4c5808938f244f4f8a35e121a3a4a862588b752d
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>